The present invention relates generally to an analyzer having a charge coupled device based emission spectrometer for ultra-high purity gas analysis.
For many years, gaseous emission spectroscopy has been used for the analysis of nitrogen in argon (see e.g., U.S. Pat. No. 3,032,654). A commonly used emission source for this technique is a low-energy argon plasma, also known as a silent electric discharge (SED). This technology has improved over the years to lower the limit-of-detection (LOD) to single digit parts-per-billion (ppb) levels; for example, through the use of electro-optical modulation (see, e.g., U.S. Pat. No. 5,412,467). Further improvements in sample cell design, electronics, and the microprocessor platform have led to the current generation of spectroscopic analyzers. The current practice of using multiple detectors and optical filters allows for the simultaneous analysis of multiple impurities if suitable emission wavelengths can be found.
The block diagram in FIG. 1A shows the emission and detection systems utilized in connection with early analyzers that perform conventional emission spectroscopy. Similarly, FIG. 1B is a block diagram for a state-of-the-art analyzer design using electro-optic modulation, as described in U.S. Pat. No. 5,412,467. In both types of systems, a high voltage transformer 1 powers a light source 2 containing a gaseous sample to be analyzed. The gases are excited by the voltage to produce optical emission lines (an emission spectrum) characteristic of each gas (impurity) in the sample. Narrow bandpass optical filters 3 isolate the strongest emission line corresponding to each impurity. Photomultipliers (PMTs) 5 convert the light output from each impurity to a current which is amplified by a frequency selective amplifier, either a fixed amplifier 6a as in FIG. 1A or a tuned amplifier 6b as in FIG. 1B, and readout 7. The conventional system uses a chopper wheel 4 to interrupt (or modulate) the light to the PMT. Whereas, the electro-optic modulation system uses a frequency doubler 8 and variable frequency oscillator 9 to modulate the light to the PMT.
To date, each generation of emission spectrometer has shared a common detection scheme. The emission line of the impurity of interest is isolated by a narrow bandpass optical filter and converted to an electrical signal through the use of a photomultiplier tube. The PMT has been the detector of choice for numerous applications in low light level spectroscopy due to the inherent high electronic gain possible through the use of the PMT. In addition to sensitivity, the PMT is also rugged, reliable, low cost, and stable over long periods of time. These are important attributes when used in a continuous-use application, such as emission spectroscopy. However, PMTs do pose several problems when used as detectors for emission spectroscopy. PMTs are comparatively large devices by today""s standards, particularly when several PMTs must be used in a single analyzer. Although PMTs are low cost, the high-quality narrow bandpass filters are not, especially when several filters are needed. Moreover, the narrow bandpass filters, which isolate the emission line of interest for a given impurity, also prevent evaluation of the background light level at the wavelength chosen for analysis.
The background light level at the impurity emission wavelength of interest can change for a variety of reasons, such as changes in temperature, sample gas pressure, excitation conditions, or other impurities entering the discharge. It is extremely difficult to distinguish background light level shifts from a changing concentration of the impurity of interest when only the emission intensity at the wavelength of interest is known. Shifts in background light level can result in problems with long term baseline drift, nonlinear calibration curves, and cross sensitivity to other impurities. These are all serious problems when attempting to perform impurity analysis on impurities with measurements in the parts per billion.
FIG. 2 illustrates the problem inherent in using the PMT and optical filter approach. FIG. 2 shows six emission spectra labeled A-F; that respectively correspond to 86, 56, 38, 25, 9 and 0 ppb concentrations of moisture (water vapor) in an argon sample gas. Each spectrum shows the region of the ultraviolet (UV) spectrum where both moisture and nitrogen have characteristic emission lines. Note that the addition of moisture causes a rise in the baseline light level, particularly in the region of the spectrum (333-360 nm) where nitrogen characteristically emits. If a PMT and optical filter are used, this increased light level could be interpreted as coming from a nitrogen impurity, resulting in an erroneously high concentration of nitrogen being reported. However, if the baseline light level shift is evaluated properly, the fact that no nitrogen emission peak is present can be correctly determined, and hence the nitrogen concentration is actually zero. The same argument applies to baseline shifts due to other factors, as mentioned above, which show up as noise and drift in the analytical results if not taken into account. Two approaches have been proposed to address the problem of changing baseline light level.
First, a separate PMT detector can be dedicated to determining the baseline emission light level rather than analyzing for an impurity. This is done by choosing a narrow bandpass filter that isolates a wavelength region of the sample gas emission spectrum close to, but not including, the impurity emission lines of interest. The analyzer then uses the ratio of the signal from the PMT measuring the impurity emission and the signal from the PMT monitoring the baseline. This approach eliminates many of the problems of the baseline emission light level. However, this technique is more complicated and requires either an additional PMT and optical filter or a reduction in the number of impurities which can be detected.
In the second approach, the baseline drift and some of the nonlinearity in the calibration curve of the analyzer are compensated for mathematically. The application of such a correction to each impurity analysis is implemented as part of the operating program of the analyzer. However, this approach is only possible if the nonlinearity is well characterized from previous experimental work.
The ability of a charge coupled device (CCD) array to easily evaluate the entire region of the spectrum of interest makes them an attractive detector choice for a number of spectroscopic methods. CCD arrays have been used in place of PMTs and narrow bandpass filters for spectroscopic applications for a number of years and small, low-cost, commercial units are available. The best known units are used for inductively coupled plasma (ICP) emission spectroscopy. These applications are well understood, but involve the use of very intense emission sources, typically ICP or microwave sources. These emission sources are far more intense and more energetic than the low-level emission sources in gas emission analyzers.
CCD arrays consist of an array of detector elements (pixels), each of which is a photodiode. However, CCDs lack the inherent high gain capability of a PMT. In this respect, the pixels act like photographic film. Low light images can be captured using longer integration times, much like a long exposure time is used with a conventional camera. However, long integration times worsen a problem inherent to CCD arrays; the so-called dark or thermal noise. If an array is left in complete darkness, it will generate a unique noise signature that is a function primarily of integration time and temperature. Managing this changing noise signature is key to using this technology when low intensity sources are to be detected.
Because of this dark noise problem, a brighter emission is needed from the impurity of interest in order to generate a useable signal from the CCD array detector. Heretofore, applications that normally use low light level emission sources with a PMT detector, such as gaseous emission spectroscopy, have required a brighter emission source if a CCD detector is to be used. Generally, a much more complicated power source is needed to achieve this brighter emission. This can significantly increase the cost, size, and complexity of the analyzer.
Therefore, a need exists for an analyzer, such as a gaseous emission spectrometer, having a CCD detector array that uses a simple low light level emission source, such as a silent electric discharge.
It is therefore an object of the present invention to provide an improved gaseous emission spectrometer.
It is a further object of the present invention to perform gaseous emission spectroscopy using a low-level emission source and a CCD detector array.
It is another object of the present invention to produce a small, low-cost, rugged analyzer for performing gaseous emission spectroscopy.
Other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification and the drawings.
To attain the above-mentioned objects, the invention provides an analyzer for performing gaseous emission spectroscopy. The analyzer has an analytical cell containing a gas sample for spectrum analysis. The analytical cell is preferably a microcell. A transformer provides a voltage to the gas sample in the analytical cell sufficient to produce a low-level emission source from the gas sample. The low-level emission source can be a silent electric discharge. A spectrometer then detects the emission spectrum from the low-level emission source. The spectrometer uses a charge coupled device array as the detector. A computer is used to control the analyzer and process the emission spectrum detected by the spectrometer. The computer subtracts a dark spectrum representing thermal noise from the charge coupled device detector from the emission spectrum. The computer also uses a calibration curve to calculate an impurity concentration for various impurities in the gas sample. A fiber-optic cable is used to couple the light emitted from the low-level emission source into the spectrometer. This analyzer can be used to analyze an ultra-high purity gas sample provided to the analytical cell in a continuous flow.
Another embodiment of the invention is an analyzer wherein the spectrometer generates an initial dark spectrum and the computer subtracts the initial dark spectrum from each emission spectrum.
A further embodiment of the invention is an analyzer wherein the computer monitors the temperature of the spectrometer and controls the spectrometer to generate an updated dark spectrum when a predetermined temperature change occurs. The computer then subtracts the updated dark spectrum from the emission spectrum.
A further embodiment of the invention is an analyzer wherein the computer dynamically determines and masks hot pixels in the charge coupled device detector, so that the masked hot pixels are not used in the spectrum analysis.
To further attain the above-mentioned object, the invention also provides a method of processing emission spectra from an analyzer having a charge coupled device array based gaseous emission spectrometer and a low-level emission source. The method first acquires a dark spectrum from the charge coupled device array. This dark spectrum represents thermal noise from the charge coupled device array. The dark spectrum is acquired by measuring the output of the charge coupled device array without incident light. A sample spectrum is acquired from the low-level emission source using the charge coupled device array. The low-level emission source is produced from a gas sample in an analytical cell. The dark spectrum is subtracted from the sample spectrum to obtain a corrected sample spectrum. A baseline is then determined for the corrected sample spectrum. Next, the emission peak and baseline areas for the emission peak region of the corrected sample spectrum are integrated. The baseline area is subtracted from the emission peak area to obtain a peak area. This peak area is then converted into an impurity concentration. This conversion uses a calibration curve to calculate the impurity concentration for various impurities in the gas sample. The low-level emission source is preferably a silent electric discharge. The analytical cell is preferably a microcell. A fiber-optic cable is used to couple the light emitted by the low-level emission source into the spectrometer. This method can be used to analyze an ultra-high purity gas sample provided to the analytical cell in a continuous flow.
Another embodiment of the invention is a method wherein the dark spectrum acquiring step acquires the dark spectrum when the spectrometer exceeds a predetermined temperature change from the temperature at which the dark spectrum was last acquired.
A further embodiment of the invention is a method wherein the dark spectrum subtracting step determines and dynamically masks hot pixels in the charge coupled device array, so that the masked hot pixels are not used in the spectrum analysis.